When samples are analyzed by various x-ray techniques, such as x-ray diffraction, it is desirable that the dimensions of the x-ray beam hitting the sample be on the order of the sample size, or of the order of the spot on the sample to be examined. This criteria on beam size is important because it maximizes spacial resolution, while minimizing background noise produced by unwanted photons. In many cases, for example in the case of x-ray diffraction of protein crystals, sample sizes are very small, and conventional x-ray diffraction equipment does not function efficiently. When traditional laboratory x-ray sources are used to analyze such small samples, beams of appropriate size are typically obtained by collimation methods. This includes such things as passing the x-ray beam through pin holes cut into x-ray absorbing materials such as lead. Because low beam divergence is also desirable, these pin holes must be placed a significant distance away from the source. This means that the solid angle of collection from the source is quite small. This in turn results in a very low intensity beam reaching the sample. One significant disadvantage of a low intensity beam is that measurement times can be extremely long. For some samples this is merely an inconvenience. However, for samples like protein crystals which have relatively short life times, this extended period of analysis can render the analysis technique useless. In all cases, extended measurement times lead to a decrease in the signal-to-noise ratio. Also, it is important for commercial analysis operations to maximize the sample through-put by minimizing analysis time. Shorter analysis times can thus lead to substantial financial rewards.
It is known in the art that to obtain more x-rays from a source, a larger spot size on the anode is required. Thus, conventional wisdom dictates that in order to decrease power transmitted to a sample, either with or without an optic, a more powerful source with a larger spot size should be used. A general rule that is followed is that the source spot size should be the size of the sample being analyzed.
It is known to the art that single hollow glass capillaries can form x-ray beams of very small dimensions see for example P. B. Hirsch and J. N. Keller, Proc. Phys. Soc. 64 369 (1951). Tapering these single capillaries to further limit output spot size is also known to the art see E. A. Stern et. al.Appl. Opt. 27 5135 (1988). However, both these devices only capture x rays from a very small portion of the source. Thus, their use also leads to x-ray beams of less intensity than is desired. Yet another disadvantage of the tapered devices is that the minimum x-ray spot size is located right at the tip of the device. This places strict limitations on the positioning of a sample. In addition, these single tapered capillaries can only form a small spot with considerable divergence. Often times for diffraction experiments, a parallel beam is desirable.
Also known to the art are multi-fiber polycapillary x-ray optics. These devices form a particular class of a more general type of x-ray and neutron optics known as Kumakhov optics. See for example U.S. Pat. No. 5,192,869 to Kumakhov. Disclosed in this patent are optics with multiple fibers which are designed to produce high flux quasi-parallel beams.
Although these optics can capture a large solid angle of x-rays from diverging sources, their potential for capturing from a small spot source or for forming small dimension output beams is limited by the relatively large outer diameter of the individual polycapillary fibers. The outer diameter of the fibers is on the order of 0.5 millimeters. Because of the fiber outer diameter these multi-fiber optics have a minimum input focal length roughly 150 millimeters. The critical angle for total external reflection at 8 keV for glass is four milliradians. Effective transmission after many reflections is obtained only if the photons are approximately one-half the critical angle. So using 0.5 mm diameter fibers, geometry shows that with a source as small as 100 .mu.m, the source-optic distance should be at least 150 mm for the outer channels to transmit effectively. Because of this relatively long input focal distance to capture a large angular range of x-rays from the source the input diameter needs to be relatively large which in turn constrains the minimum diameter and maximum intensity (photons/unit area) of the output beam. The minimum beam diameter for a multi-fiber polycapillary optic with a 0.15 radian capture angle which forms a quasi-parallel beam is on the order of 30 millimeters. These optics are thus not appropriate to produce the intense small diameter x-ray beams needed for small sample diffraction experiments such as protein crystallography. For focusing optics, because of the fiber diameter, the minimum focused spot sized has a diameter on the order of 0.5 millimeters.